DESCRIPTION: The hair cell and its afferent and efferent synapses provide a remarkable system for studying neurotransmitters. The innervation patterns are relatively simple and well defined, yet the processes mediated by these neurotransmitters challenge the limits of our understanding of neurobiology. To accomplish our long-term goal of understanding afferent and efferent transmission in hair cell organs, we have taken a pharmacological approach to characterize molecular and cellular processes in the intact Xenopus lateral line organ and guinea pig cochlea. With this approach we have made significant progress toward understanding the action of two efferent neurotransmitters (acetylcholine (ACH) and calcitonin gene-related peptide (CGRP)) and in identifying the hair cell afferent transmitter. In the previous project period, we found that CGRP acts like ACH to decrease sensitivity to mechanical stimulation, but differs from ACH in increasing spontaneous discharge. In the coming project period, we will follow up this finding to determine the actions and interactions of CGRP and ACH as efferents transmitters and will complete work to identify a hair cell afferent neurotransmitter candidate we have purified from retina and inner ear. Specifically, we will 1) determine the mechanism of CGRP'S action as an efferent neurotransmitter in the lateral line organ; 2) determine the mechanism of ACH's action as an efferent neurotransmitter in the lateral line organ and its interaction with CGRP; 3) determine he action of CGRP in the mammalian cochlea; and 4) identify the neurotransmitter released by the hair cell to excite afferent nerve fibers. The development of an understanding of neurotransmitters in hair cells organs will continue to provide insight into basic neurobiological mechanisms. The development of an understanding of neurotransmitters in hair cells organs will continue to provide insight into basic neurobiological mechanisms. For instance, the identification of our "new" neurotransmitter candidate can serve as a probe for all of neurobiology. As we advance our understanding of the role of CGRP as an efferent transmitter and consider its interactions with ACH, we should enhance the ability to probe cochlear function and pave the way for future analysis of these complexities at the biophysical and molecular levels.